1. Field of the Invention
The present invention relates to methods for obtaining improved spatial and energy resolution in room temperature HgI2 radiation detectors, and more specifically, it relates to a method for lithographically patterning HgI2 crystals.
2. Description of Related Art
Semiconductor materials exhibit a band gap between their valence and conduction bands of typically a few eV. Because this energy gap is so low, as the temperature of the crystal is increased, electrons are thermally excited and easily move from the valence band to the conduction band. The electrical properties of these materials, therefore, are affected not only by the movement of electrons into the conduction band but also by the formation of vacant sites or xe2x80x9cholesxe2x80x9d in the valence bands left behind by the departing electrons. Both can conduct current.
Holes also may be created by the interaction of energetic radiation, such as X-rays, gamma rays, and the like, with intrinsic semiconductors and therefore one should be able to use these materials as detectors for measuring high-energy radiation. In fact, high-resistivity semiconductor radiation detectors are widely used for detecting ionizing radiation due to their ability to operate at room temperature, their small size and durability. Such detectors are used in a wide variety of applications, including medical diagnostic imaging, nuclear waste monitoring, industrial process monitoring, and space astronomy. Ionizing radiation includes both particulate radiation such as alpha or beta particles and electromagnetic radiation, such as gamma or x-rays.
If all the electrons and holes generated by the ionizing radiation reach their respective electrodes (i.e., the electrons reach the anode and the holes reach the cathode), the output charge signal will exactly equal the charge from the ene deposited within the crystal by the radiation. Because the deposited charge is directly proportional to the energy of the ionizing radiation, the semiconductor detector provides a means for measuring the energy of the ionizing radiation.
Room temperature detectors, however, suffer from a serious drawback. Because of limitation in the transport properties of the bulk semiconductor crystal, some of the electrons and, more particularly, some holes are generally lost by being trapped as they move toward the respective electrodes under the influence of the external electrical field. This is particularly evident for semiconductors wherein the transport properties of one carrier type (e.g., electrons) are much better than those of another type (in this example the xe2x80x9cholesxe2x80x9d). Therefore under such circumstances the amplitude of the output charge signal becomes dependent on the position within the crystal at which the ionizing radiation is absorbed. Generally speaking, the amplitude is less than the charge deposited by the ionizing radiation and results in a corresponding reduction of energy measurement accuracy, poor resolution, and reduced peak efficiency. This loss (or trapping) of charge in a radiation detector results in distorted and asymmetrical spectral peak shapes known as xe2x80x9chole tailingxe2x80x9d or xe2x80x9chole trapping.xe2x80x9d
The inability to eliminate xe2x80x9cholexe2x80x9d drift current is a major impediment for the use of room temperature semiconductors as detectors. Gamma-ray spectroscopy is particularly encumbered because pulse height spectra produced by these devices are distorted by this process. Mono-energetic gamma rays produce charge signal responses of different pulse height because the total combined distance drifted by the electrons and holes is dependent on the position of gamma-ray interaction. This phenomenon is well known in the prior art and has been described by many researchers. It is widely understood to be the major deficiency limiting the effectiveness of room temperature semiconductor materials.
Due to the deleterious effects of hole-trapping in semiconductor detectors, much effort has gone into attempting to solve this problem. U.S. Pat. Nos. 4,253,023 and 4,996,432 recognized the problem and proposed early remedies. The first of these included a method to de-convolute the contribution of the electron motion from the acquired signal. The second approach proposes a method relying upon use of a thick crystal and a crystal orientation placing the detector anode surface facing the source of radiation, thereby reducing the positional dependence of the radiation interaction with the crystal and restricting it only to that part of the crystal immediately behind the anode. Neither of these approaches directly addresses the problem of eliminating hole-trapping.
U.S. Pat. No. 5,677,539 provides another approach and a comprehensive review of much of the pertinent prior art. A particularly relevant approach, described therein, employs an anode patterned into an interleaving grid structure, with the, cathode remaining planar. (See, e.g., P. N. Luke, xe2x80x9cUnipolar Charge Sensing with Coplanar Electrodesxe2x80x94Application to Semiconductor Detectors,xe2x80x9d IEEE Tran. Nucl. Science, vol. 42, No. 4, at pages 207-213 (1995)). In this approach, one set of anode grids is maintained at a slightly higher voltage than the other. A train of signal conditioning electronics is connected to each set of grids, and the difference between the outputs from these trains constitutes the final output signal. With this arrangement, when the charge cloud is far from the grids, the difference-signal between the grid outputs is zero. As the cloud approaches the grids, the induced charge on one grid rises rapidly, while the charge induced in the other grid drops rapidly. The difference signal is then a measure of the full charge in the electron cloud, independent of the position of the ionizing event.
This approach, however, also suffers from various drawbacks. First, the grid structure is relatively complex and would be difficult, if not impossible, to use in detector arrays. Second, the grids require two separate amplifying chains, plus a difference amplifier, adding significantly to the complexity and cost of manufacture. This circuitry also would be difficult to implement in the multichannel type integrated circuits needed in detector array structures.
A relatively simpler structure is a variation on a technique devised by Frisch for use in gas detectors. The hole-trapping phenomenon observed in semiconductor detectors is analogous to the trapping behavior of positive ions in gas detectors. Frisch proposed, and later developed, detectors that contained a grid of conductive wires between the two electrodes of a conventional gas detector.
In the Frisch grid type detector, the signal is measured between the anode and the grid. The negative charge carriers, electrons in the case of a semiconductor detector, usually drift all of the way to the anode. Thus, any radiation interactions occurring between the cathode and the grid will produce electrons that drift past the grid and on to the anode. When the electrons drift across the gap between the grid and the anode, they will induce a current that is due solely to the motion of electrons. The current induced by positively charge carriers (xe2x80x9cholesxe2x80x9d in the case of semiconductors) traveling in the opposite direction is shielded by the grid. The main advantage, then, of the Frisch grid is that the signal produced from the device is independent of the position of interaction between the cathode and grid (completely solving the hole trapping problem in the cathode to grid region).
An approach to reducing hole-trapping in a semiconductor detector using a modified Frisch grid approach has been proposed by D. S. McGregor. McGregor""s scheme, described as an xe2x80x9cetch trenchxe2x80x9d device, entailed building a Frisch Grid device on a semiconductor crystal and placing electrodes at the bottom of these trenches. The primary disadvantage of McGregor""s etch-trench design is that it is difficult to execute. In other words, it might be very difficult to produce the requisite trenches, particularly since the fabrication technology for room temperature semiconductor materials is not well developed at this time. Therefore, while a Frisch-grid type device design would work very well in a room temperature semiconductor detector, it suffers from the fundamental drawback of being difficult to construct. Such a device, owing to the difficulty of placing a conductive grid inside a semiconductor crystal would be inherently complex and expensive.
A third approach is discussed in U.S. Pat. No. 5,677,539. This invention takes advantage of the principle that a significant reduction in tailing in a semiconductor detector can be attained by a novel arrangement of electrodes that share induced charge from ionizing events in the detector, that properly shape the electric field, and that focus charge collection toward a small electrode.
The detector includes three electrodes formed on the surface of a semiconductor crystal. The crystal has a plurality of sides; it preferably has a thickness of at least about 0.5 mm and is preferably formed from a semiconductor material having a higher mobility-lifetime product for one electronic charge carrier compared to the other. The first electrode is a bias electrode, which preferably covers the entire surface of one side of the crystal. At least one signal electrode having a small area is preferably formed on the opposing side of the crystal from the bias electrode. A control electrode is preferably disposed on the same side containing the signal electrode.
In particular, the control electrode is formed on the same side of the semiconductor crystal as the signal electrode (anode), and the bias electrode (cathode) covers substantially the entire surface of the opposite side of the crystal. The semiconductor crystal is formed from CdZnTe or CdTe. In the simplest configuration, the anode is a small contact point located near the center of the electron-charge-collection side of the crystal. The anode is coupled to ground through a large-value resistor and to external signal circuitry. The cathode is coupled to a voltage source that maintains the cathode at a negative voltage level relative to the anode. Preferably, the control electrode is much larger in area than the anode and forms a single ring surrounding the anode. The control electrode is maintained at a voltage level that is negative with respect to the anode, but generally not more negative than the cathode.
This approach relies on a geometric construction of the various electrodes to shape the electric field within the crystal in order to direct and accelerate the more mobile of the charge carriers toward the appropriate collecting electrode.
By operating in this manner the less mobile of the charge carriers, those that are xe2x80x9ctrappedxe2x80x9d, arrive typically too late to contribute to the analysis. However, while much reduced, some small percentage of these carriers still do contribute to the signal.
The performance of gamma-ray and x-ray spectrometers fabricated from HgI2 is substantially limited by hole trapping. The ability to pattern arbitrarily sized and positioned electrodes allows for enhanced performance via the so-called xe2x80x9celectron-onlyxe2x80x9d behavior, which substantially increases detector energy resolution. Hole trapping distorts the current pulse generated by the absorption of an x- or gamma-ray, which reduces the ability of the detectors to spectrally resolve the unique radiological emissions of a wide variety of isotopes. Degradation in the capability of the spectrometers to resolve energies of x-rays and gamma-rays reduces the marketability of the devices.
For imaging devices, the spatial resolution is limited by the pixel size of each individual detector element. It is desirable that pixel size be significantly decreased as compared to the current method of shadow mask deposition. The ability to pattern substantially denser arrays of smaller pixels will improve the spatial resolution of these imaging devices.
It is an object of the present invention to provide a method using a photolithographic process for forming patterns on HgI2 surfaces.
It is another object of the present invention to provide a method for smoothing HgI2 surfaces.
Still another object of the present invention is to provide a method for producing trenches in HgI2.
It is another object of the present invention to provide a sublimation process for trench etching to produce etched-trench devices with enhanced electron-transport-only behavior.
Another object of the invention is to provide a lithographic process for defining a metal sublimation mask and electrodes to substantially improve device performance by increasing the realizable design space.
These and other objects of the invention will be apparent to those skilled in the art based on the teachings herein.
The invention patterns HgI2 surfaces with a new photolithographic process. The process is used to define metal sublimation masks and electrodes on HgI2. Design space is increased to substantially improve device performance. Techniques for smoothing HgI2 surfaces and for producing trenches in HgI2 are provided. A sublimation process is described which produces etched-trench devices with enhanced electron-transport-only behavior.
To deposit a continuous Pd (or other metal) film on a given substrate (e.g. mercuric iodide), the film thickness must be about as thick as (or greater than) the surface roughness. Rapid agitation of the etch solution produced smooth surfaces. The invention provides agitation parameters that produce optimal smoothness.
Materials used for both electrode and sublimation mask metal layers include palladium, gold, carbon, indium-tin-oxide, chromium, tantalum and platinum. The films are deposited using techniques such as sputtering and electron beam deposition. Although sputtering will generally not cause substrate heating above 50-100xc2x0 C. for plasma powers less than 200 W, it was necessary to cool the substrate to optimize adhesion.
Shadow masks are typically used to pattern HgI2 because acetone and/or other solvents used to remove the photoresist after patterning attack HgI2 rapidly. HgI2 cannot withstand the high temperatures typically required for photoresist baking. Shadow masks, however, are quite limiting in terms of minimum pattern size and freedom of design. Fine line widths of guard electrodes are required for electron-only operation of HgI2 spectrometers, and shadow masks are often not suitable for generating them.
The present invention provides a new lithography process that is compatible with HgI2. A low-temperature photoresist is employed in the new process. Solvents are not used for dissolving the photoresist after the metal etching step. A high-temperature bake was not used. The palladium etchant was chosen such that a soft-bake was sufficient to make the photoresist a suitable mask for the Pd etch step. Without the hardbake step, the photoresist removal can subsequently be promoted by a flood exposure step after the Pd etching.
Trenches were etched by a sublimation process after the first Pd metallization and patterning steps. In the present invention, sublimation is governed by temperature only and not by gas flow as long as the pressure is kept somewhat below 11 mTorr. In addition to being temperature dependent, the sublimation rate was also found to be dependent on the particular etchant used for the Pd patterning, with an increase in Br content generating higher etch rates and a redder surface. After the sublimation step was completed, the entire surface was metallized again and patterned with collection and grid electrodes.